Aluminum is conventionally produced in electrolytic reduction cells or smelting pots which include an electrolytic bath comprising molten aluminum fluoride, sodium fluoride and alumina, a cathode, and consumable carbon anodes. The energy and cost efficiency of aluminum smelting can be significantly reduced with the use of inert, non-consumable and dimensionally stable anodes. Replacement of traditional consumable carbon anodes with inert anodes allows a highly productive cell design to be utilized, and may provide environmental benefits because inert anodes produce essentially no CO2 or CF4. Some examples of inert anode compositions are provided in U.S. Pat. Nos. 5,794,112, 5,865,980, 6,126,799, 6,217,739, 6,332,969, 6,372,119, 6,416,649, 6,423,195 and 6,423,204, which are incorporated herein by reference.
During aluminum smelting operations, deleterious impurities such as sulfur, iron, nickel, vanadium, titanium and phosphorous may build up in the electrolytic bath. For example, in inert anode cells, sulfur species can build to higher concentrations in the bath because it is no longer removed as COS or other sulfur-containing species as in consumable carbon anode cells. The presence of sulfur or other multi-valence elemental impurities in the bath causes unwanted redox reactions which consume electrical current without producing aluminum. Such impurities can significantly reduce the current efficiency of the cells. Sulfur species have a high solubility in the bath and act as oxidizing agents to react Al to form Al2O3. This can cause unwanted back reaction of the aluminum which also reduces the current efficiency of the cell. Furthermore, sulfur, iron, nickel and other impurities in the bath can lower the interfacial energy between the bath and the molten pad of aluminum formed in the cell, thereby reducing coalescence or promoting emulsification of the surface of the aluminum pad.
The present invention has been developed in view of the foregoing, and to address other deficiencies of the prior art.